This invention relates generally to modulators and more specifically concerns a method and apparatus for phase-modulating a carrier signal with a high degree of linearity between the modulating input signal and the phase of the output carrier signal.
A previous technique for phase-modulating a carrier signal was to generate signals representing sine and cosine waves of the modulating signal and multiply these signals with two quadrature components of the carrier wave. The resulting two signals were subtracted to produce a carrier wave output signal that has a phase angle which is linear with respect to the modulating signal. An example of this technique is disclosed in U.S. Pat. No. 2,635,226. A disadvantage of this technique is that in the present state of the development of sine and cosine modules, they are basically two-quadrant devices. Hence, a phase modulator built in accordance with this prior art technique is limited to plus or minus 90.degree. deviation. It is therefore an object of this invention to provide a phase modulating technique that is not limited to plus or minus 90.degree. deviation. It is a further object of this invention to provide a phase modulating technique in which the linearity between the modulating signal and the difference in phase between the input carrier signal and the output modulated carrier signal is greatly improved.